Stimulation‐evoked release of [3H]‐noradrenaline by 1, 10 or 100 pulses and its modification through presynaptic α2‐adrenoceptors

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Abstract
1 Mice isolated vasa deferentia were loaded with 1-[7, 8-3H]-noradrenaline and subsequently field stimulated with 1, 10 or 100 pulses (2 ms pulse width, 1 Hz). The tritium overflow was separated into [3H]-noradrenaline and its 3H-metabolites. 2 The resting release of tritium contained about 7% [3H]-noradrenaline, 33% [3H]-3, 4-dihydroxyphenylglycol ([3H]-DOPEG) and 60% 3H-non-catechols with usually less than 1% [3H]-dihydroxymandelic acid ([3H]-DOMA). The proportion of the tritium as [3H]-noradrenaline increased with stimulation train length to 35% with 100 pulses; this increase in [3H]-noradrenaline was associated with falls in [3H]-DOPEG and 3H-non-catechols. Generally the proportional increase in [3H]-noradrenaline on stimulation was about 10 × total tritium when compared with the resting release. 3 The fractional release of [3H]-noradrenaline per pulse was independent of train length, averaging about 6 × 10−6. This was reduced by the α2-adrenoceptor agonist clonidine (0.3–30 nm) with an IC5o of 4.8 nm (10 pulses at 1 Hz). 4 The α2-adrenoceptor antagonist, yohimbine (10–100 nm), did not alter the fractional release of [3H]-noradrenaline elicited by 1 pulse. The antagonist did not change the amount or composition of the resting or evoked tritium overflow. However, yohimbine (l-100 nm) increased the fractional release of [3H]-noradrenaline per pulse for trains of 10 or 100 pulses (1 Hz) in a concentration-dependent fashion. An increase above controls was significant only with 100 pulses and yohimbine, 30 nm. 5 The results show that the release of noradrenaline during trains of pulses in the mouse vas deferens can be regulated through presynaptic α2-adrenoceptors. There was no evidence of inhibition by noradrenaline of its own release following a single pulse.
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